<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .boxa{
            height:100px;
            width:100px;
            background-color: aqua;
            position: absolute;
            left:60px;
            top:100px;
        }
        .boxb{
            height:100px;
            width:100px;
            background-color: blue;
            position: absolute;
            left:80px;
            top:100px;
            /* z-index 设置层级，默认0 auto 
               配合定位
            */
            /* z-index:10; */
        }
        .boxc{
            height:100px;
            width:100px;
            background-color: orange;
            position: absolute;
            left:100px;
            top:100px;
            z-index:-10;
        }
    </style>
</head>
<body>
    <div class="boxa"></div>
    <div class="boxb"></div>
    <div class="boxc"></div>
</body>
</html>